Today is Father’s Day, and of course I’m going to dedicate this post to my dad, who reads my posts and always sends me corrections (and is the top commenter on the blog). So to celebrate Father’s Day, I’m going to write about a species that doesn’t actually invest any parental care into their offspring: the common cuckoo. Don’t worry, this isn’t a reflection on human fathers at all, I’ve just been wanting to write about cuckoos for a while and it happened to be the right day for it today.

Common cuckoos live in Europe and Asia in the summer, and overwinter in Africa and southeast Asia. They come to Europe in April and leave for warmer climes in September. Cuckoos are well adapted to live in a number of different habitats, including forests, farmlands, and open lands and marshes.

Cuckoos are fairly large birds, measuring 32-34 centimetres from bill to tail, and a wingspan of 55-60 centimetres. They are usually grey in colour, with black bars on their underside. Female cuckoos occasionally have a brown colour morph which makes them have brown feathers all over except for the barred back on their undersides. The colour of cuckoos is thought to be a mimicry of the sparrowhawk, a predatory bird. Mimicking sparrowhawks lets cuckoos get closer to other birds’ nests – a little bird that would try and fight off a cuckoo trembles in terror at the thought of approaching a sparrowhawk.

A comparison of a cuckoo (top) and sparrowhawk (bottom)
Image By Chiswick Chap – Own work, CC BY-SA 3.0

The ability to approach other birds’ nests is key to the cuckoo’s breeding strategy. You see, cuckoos are brood parasites, meaning they lay their eggs in another bird’s nest, and then leave the unborn chick there to fend for itself. A female cuckoo flies to a nest, removes one of the host’s eggs, and then lays one of her own. She can do this in about ten seconds, and can lay up to fifty eggs in a breeding season. Her eggs resemble those of the host species – which can be any of a hundred species known to be targeted by cuckoos. The most common species targeted are the meadow pipit, the dunnock and the European reed warbler, though they’ve been reported in the nests of nearly 300 species.

The cuckoo’s eggs need to be near perfect mimics – if they aren’t, there’s a chance the host will recognize the egg and expel it from the nest. If the host doesn’t do this, it is a disaster for the poor bird’s family. You see, cuckoos try and ensure that their chicks hatch before the host chicks, so the baby cuckoo can push the other eggs out of the nest. The mother cuckoo can keep her egg inside her for an extra 24 hours before laying, internally incubating the egg so it hatches sooner and the chick is more developed when it hatches. Young cuckoos expel any ‘nestmates’ because cuckoos are much, much bigger than their hosts, and need all the host parents’ attention to get enough food. At fourteen days of age, the cuckoo chick is three times the size of an adult European reed warbler.

A European reed warbler feeds a cuckoo chick. The size difference blows my mind.
Image By Per Harald Olsen – Own work, CC BY-SA 3.0

So why do other birds put up with this? It’s a great breeding strategy for cuckoos, but the host parents could just not feed the invading chick. Apparently they just can’t help it. If hosts recognize a cuckoo egg, they’ll expel it from the nest, but once it’s hatched, they have to feed the chick. The cuckoo chick also has a cry that sounds like an entire brood of nestlings, in an attempt to garner more food from the hosts. It’s a devilish scheme, and an inventive one for cuckoos (and other birds that do this – cuckoos aren’t the only brood parasites in the animal world), but it can’t be nice for the hosts. Still, I always appreciate animal ingenuity when I come across it, so let’s celebrate the cuckoo’s clever parental strategy. Happy Father’s Day!

Cover image By Vogelartinfo – Own work, GFDL 1.2